Even though the 60Hz driving waveform is slow, when the diode switches on you will have higher speed transient. To make matters worse the peaks of the voltage waveform are clipped and contain high frequency harmonics and RFI components that can couple to the bridges trough parasitic winding capacitance in the transformer.

The article http://www.gensemi.com/appnotespdf/quik108.pdf
refers to an article by Rick Miller with pictures and data in the “Audio Amateur” of Jan ‘94. The spectra were measured with a spectrum analyzer with an RF amplifier and high pass filter between the DC supply the analyzer. very many high end products are designed with high speed soft recovery diodes now since this been know about for quite some time.

There may be some conventional diode for the Digital volume control interface. I have seen high speed diodes in several McCormack products. Steve was one of the first to do this in commercial products. i think the high speed diodes were from Harris.

Yes i see a point in favour of high speed diodes as they have less parasitic capacitance bettwen anode and cathod...and is that quality (low capacity) that is lost when you add a capacitor across the diode...

The low capacity in a diode helps to isolate fom the noisy secundary transformer...and from the noise in the mains( via internal capacity of the transformer)...from the delicat audio circuits!

If you add a capacitor acrss the capacitors...you shunt this noise to the circuit and lost the diodes(high speed) advantage!!

It's easy to talk about specs, but it's hard to find a good sounding diode.

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Schottky rectifiers turn on fast; if the power supply capacitor has
drained significantly during the half-cycle, then the turn-on surge
will 'ring' the secondary winding. This ringing may be at a lower frequency than the RFI generated internally by the rectifier.
This may be a more significant problem with a power amplifier
under heavy load than a preamp running at a constant and
very modest power level.

I have found that a snubber across the secondary winding can
greatly reduce this ringing; 0.47 uF and 100 ohms is a good starting point for experimenting. It is necessary to tune both
value for best results, and small transformers benefit the most.

A shunt capacitor across the rectifier helps to isolate the RFI
generated by the rectifier itself, but a snubber is better and
the actual values are different from the snubber used on a
transformer secondary.

I've had trouble measuring the RFI from rectifiers, even with a
100 Mhz oscilloscope. A cheap transistor AM band radio might
make a good detector. I haven't tried this.